U.S. patent number 5,750,434 [Application Number 08/423,841] was granted by the patent office on 1998-05-12 for surface polishing of silicon carbide electronic device substrate using ceo.sub.2.
This patent grant is currently assigned to Fuji Electric Co. Ltd.. Invention is credited to Shinji Ogino, Tatsuo Urushidani.
United States Patent |
5,750,434 |
Urushidani , et al. |
May 12, 1998 |
Surface polishing of silicon carbide electronic device substrate
using CEO.sub.2
Abstract
A silicon carbide substrate is dry-polished using chromium oxide
Cr.sub.2 O.sub.3, ion oxide Fe.sub.2 O.sub.3, or cerium oxide
CeO.sub.2 to obtain a good polished surface free of mechanical
defects and with less crystal distortion. Films are then formed on
the surface to create an improved electronic device.
Inventors: |
Urushidani; Tatsuo (Yokosuka,
JP), Ogino; Shinji (Kamakura, JP) |
Assignee: |
Fuji Electric Co. Ltd.
(JP)
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Family
ID: |
26427779 |
Appl.
No.: |
08/423,841 |
Filed: |
April 18, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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214637 |
Mar 18, 1994 |
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Foreign Application Priority Data
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Mar 22, 1993 [JP] |
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5-86681 |
May 20, 1993 [JP] |
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5-141343 |
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Current U.S.
Class: |
438/478;
257/E21.237; 257/E21.461; 438/492; 438/690; 438/931; 438/959;
438/974 |
Current CPC
Class: |
H01L
21/304 (20130101); H01L 29/1608 (20130101); H01L
29/47 (20130101); H01L 21/02378 (20130101); H01L
21/02529 (20130101); H01L 21/02658 (20130101); Y10S
438/931 (20130101); Y10S 438/974 (20130101); Y10S
438/959 (20130101) |
Current International
Class: |
H01L
21/36 (20060101); H01L 21/304 (20060101); H01L
21/02 (20060101); H01L 021/306 () |
Field of
Search: |
;156/645.1
;437/77,175,100,904 ;438/690,931,959,974,478,492 ;148/DIG.148 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Suzuki et al, "C-V Characteristics of SiC Metal-Oxide-Semiconductor
Diode with a Thermally Grown SiO.sub.2 Layer", Appl. Phys. Lett.
39(1), 1 Jul. 1981, p. 89. .
"Step-Controlled VPE Growth of SiC Single Crystals at Low
Temperatures", Kuroda, Naotaka et al., Extended Abstracts of the
19th Conference on Solid State Devices and Materials, Tokyo 1987,
pp. 227-230. .
"Mechanochemical Polishing of Single-Crystal Silicon Carbide",
Kikuchi, Masao et al., The Corrected Papers for the 1990 Autumn
Scientific Lecture Meeting; pp. 327-328. .
M. Kikuchi et al. "Mechanochemical Polishing of Silicon Carbide
Single Crystal with Chromium (III) Oxide Abrasive." Journal of the
American Ceramics Society, 75(1), (1992) pp. 189-194..
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Primary Examiner: Bowers, Jr.; Charles L.
Assistant Examiner: Whipple; Matthew
Attorney, Agent or Firm: Rossi & Associates
Parent Case Text
This is a Division of application Ser. No. 08/214,637 filed Mar.
18, 1994, abandoned.
Claims
We claim:
1. A method of forming an electronic device comprising the steps
of: dry-polishing a surface of a silicon carbide substrate with a
polishing means consisting essentially of cerium oxide CeO.sub.2 ;
and forming a silicon carbide epitaxial film on a front surface of
the silicon carbide substrate.
2. A method of forming an electronic device as claimed in claim 1,
further comprising the steps of forming a first metal film
electrode on the silicon carbide epitaxial film and forming a
second metal film electrode on a back surface of silicon carbide
substrate.
3. A method of forming an electronic device as claimed in claim 1,
further comprising the steps of forming an insulation layer on the
silicon carbide epitaxial film, forming a first metal film
electrode on the insulation layer, and forming a second metal film
electrode on a back surface of the silicon carbide substrate.
4. A method of forming an electronic device as claimed in claim 1,
wherein a breakdown voltage of the electronic device is not less
than 400 volts.
5. A method of forming an electronic device as claimed in claim 2,
wherein a breakdown voltage of the electronic device is not less
than 400 volts.
6. A method of forming an electronic device as claimed in claim 3,
wherein a breakdown voltage of the electronic device is not less
than 400 volts.
Description
FIELD OF THE INVENTION
The present invention relates to an electronic device and, in
particular, to a method of surface polishing of a silicon carbide
substrate used in an electronic device.
BACKGROUND OF THE INVENTION
There have been recent attempts to improve the high-frequency and
high power performance of silicon-based power devices. The level of
performance of such devices, however, is approaching its limit. In
addition, power devices are often used in conditions where high
temperatures or radiation is present. Silicon devices, however,
cannot be used under such conditions. New material must therefore
be utilized to provide electronic devices with higher
performance.
Since silicon carbide has a wide prohibited band (6H type: 2.93
eV), it can control electric conductivity well and provide high
radiation-resistance at high temperatures. Silicon carbide has a
dielectric breakdown electric field that is one order of magnitude
more than of silicon, which makes it possible to utilize silicon
carbide to high voltage-proof devices. In addition, its electron
saturation drift velocity, which is twice that of silicon, makes it
possible to utilize silicon carbide to high-frequency high power
control.
In order to utilize the good material characteristics of silicon
carbide in power devices, a substrate of silicon carbide is
epitaxially formed and a metal or an oxide film is formed on the
substrate to make an electronic device after the surface of a
silicon carbide substrate has been finished sufficiently to make it
a mirror surface. However, since the silicon carbide substrate has
a hardness of nine, it is chemically stable, and is not affected by
acid or alkali, diamond grindstones must be used to polish the
silicon carbide substrate. Such polishing causes fine flaws or
cracks to be created or causes the crystal structure on the surface
of the silicon carbide substrate to be disturbed. Such defects must
be removed by means of etching. Although etching eliminates the
disturbance of the crystal structure, however, it does not remove
flaws or cracks. In fact, the number of such defects are increased
by etching. If an oxide or a metal film is formed on the silicon
carbide substrate, electric fields concentrate on the defective
parts (such as flaws or cracks) created by polishing, and cause the
characteristics of the electronic device to deteriorate.
Given the above realities, it is an object of the invention to
provide a good silicon carbide electronic device by providing a
silicon carbide substrate with a smooth surface and a crystal
structure free of disturbances.
SUMMARY OF THE INVENTION
The above object is achieved by providing an electronic device that
uses a silicon carbide substrate which has a silicon carbide
substrate and a film formed thereon, wherein the silicon carbide
substrate has its surface dry-polished by a polishing means made up
of at least one of the following: chromium oxide Cr.sub.2 O.sub.3,
cerium oxide CeO.sub.2, and iron oxide Fe.sub.2 O.sub.3. The film
is formed on the dry-polished surface of the silicon carbide
substrate.
Polishing with chromium oxide Cr.sub.2 O.sub.3, cerium oxide
CeO.sub.2 or iron oxide Fe.sub.2 O.sub.3 provides a good polished
surface due to the mechanochemical effects of these substances.
Because of these mechanochemical effects, frictional heat generated
during polishing and the catalysis of these oxides cause silicon
carbide to oxidize because of the oxygen in the air, thereafter
being mechanically released. As a result, the surface of the
silicon carbide substrate is polished, being free of mechanical
defects or crystal distortion.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in greater detail with
reference to the accompanying drawings, wherein:
FIG. 1 is a sectional view of a first electronic device made in
accordance with the invention;
FIG. 2 is a sectional view of the important part of another
electronic device made in accordance with the invention;
FIG. 3 is a photograph showing the grain structure of the polished
surface of a silicon carbide substrate in accordance with the
invention;
FIG. 4 is a photograph showing the grain structure of the polished
surface of the silicon carbide substrate of a conventional
electronic device;
FIG. 5 is a histogram illustrating the distribution of the
breakdown voltage of one hundred electronic devices formed on a SiC
substrate prepared in accordance with the invention; and
FIG. 6 is a histogram illustrating the distribution of the
breakdown voltage of one hundred electronic devices formed on a SiC
substrate prepared by a conventional method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A first electronic device formed in accordance with the invention
is described below with reference to the drawings. FIG. 1 is a
sectional view of an electronic device according to this invention.
The electronic device is a Schottky diode. The Schottky diode is
manufactured by polishing a silicon carbide substrate 3, forming a
silicon carbide epitaxial film 2 on the substrate 3, forming a
nickel electrode 4 on the other main surface of the silicon carbide
substrate 3, and forming a gold electrode 1, e.g. a Schottky
electrode, on the silicon carbide epitaxial film 2.
The polishing of the silicon carbide substrate 3 will now be
explained in detail. A polishing plate with chromium oxide Cr.sub.2
O.sub.3 abrasive grains measuring 1 .mu.m distributed in phenol
resin is rotated at 100 rpm to polish a specified surface of the
substrate 3 (for example a n-type 6H-silicon carbide SiC substrate)
while the substrate 3 is inclined at an angle of 5.degree. in a
specified direction. Table 1 shows the specified surface and
direction:
TABLE 1 ______________________________________ Specified Surface
Specified Direction ______________________________________ (0001)
<1120> ______________________________________
For the sake of comparison, after polishing with silicon carbide
SiC abrasive grains measuring 150 .mu.m, silicon carbide substrates
were polished with diamond abrasive grains measuring 30 .mu.m, 3
.mu.m, and 1 .mu.m. Finally, these substrates were buffed with
diamond grains measuring 1 .mu.m. Moreover, the substrates were
simultaneously etched in a mixed gas made up of hydrogen gas with a
flow of 2 slm and hydrogen chloride HCl gas with a flow of 3 sccm
at 1200.degree. C. for 5 minutes. After that, they were observed
using a Nomarski microscope. No flaws were found on the surfaces
polished with chromium oxide Cr.sub.2 O.sub.3 (see FIG. 3), whereas
the surfaces polished with diamond abrasive grains of various sizes
eventually took on a mirror-surface finish, although groove-like
polishing flaws created by the etching process were found (see FIG.
4). In other words, fine flaws that had not been detected by the
Nomarski microscope became larger after etching with hydrogen
chloride HCl. However, no groove-like etched parts and few flaws
were found on the surfaces polished with chromium oxide Cr.sub.2
O.sub.3 and etched with HCl. Many hexagonal etching pits in the
figure were created because of transition and were not caused by
polishing.
An n-type epitaxial film with a carrier density of
2.times.10.sup.15 cm.sup.-3 and a thickness of 9 .mu.m was formed
on a silicon carbide Sic substrate (n-type: carrier density:
1.times.10.sup.18 cm.sup.-3) polished by using the above two
methods. As described above, a mixed gas including hydrogen H.sub.2
gas with a flow of 2 slm and hydrogen chloride HCl gas with a flow
of 3 sccm at 1200.degree. C. was introduced to etch the surface.
Then, a mixed gas made up of hydrogen H.sub.2 gas with a flow of 3
slm, silane SiH.sub.4 gas with a flow of 3 sccm, and propane gas
with a flow of 2.5 sccm at 1500.degree. C. for 4 hours was
introduced to form the n-type 6H-silicon carbide Sic epitaxial film
2. The epitaxially formed film had surface conditions similar to
the surface conditions present after etching, shown in the previous
figure, and the epitaxial film formed on the substrate polished
with chromium oxide Cr.sub.2 O.sub.3 had fewer flaws and a smoother
surface than the film formed epitaxially on the substrate polished
with diamond grains.
Ni was vacuum-evaporated on the back surface of the n-type
6H-silicon carbide Sic substrate 3, which was then heated at
1200.degree. C. in an Ar atmosphere for 10 minutes to obtain an
ohmic nickel electrode 4. Au was then vacuum-evaporated on the
n-type 6H-silicon carbide Sic epitaxial film 2 to obtain a gold
electrode 1, that is, a Schottky electrode.
FIG. 5 is a diagram illustrating the distribution of the breakdown
voltages for one hundred electronic devices formed on a substrate
prepared in accordance with the invention, while FIG. 6 is a
diagram illustrating the distribution of the breakdown voltages of
one hundred electronic devices formed on a substrate prepared by
conventional methods. As can be seen from the figures, the
electronic device according to the invention has a breakdown
voltage higher than that of a conventional electronic device. This
characteristic is also observed when either cerium oxide CeO.sub.2
or iron oxide Fe.sub.2 O.sub.3 is used instead of chromium oxide
Cr.sub.2 O.sub.3.
FIG. 2 is a sectional view of the important part of an electronic
device according to a different embodiment of the invention. The
illustrated device is a MOS diode. An n-type silicon carbide SiC
epitaxial film 6 is laminated on a n-type silicon carbide substrate
7, and a silica SiO.sub.2 insulation layer 5 is formed on the
epitaxial film 6.
The invention provides an electronic device that uses a silicon
carbide substrate which has a silicon carbide substrate and a film
formed thereon. The silicon carbide substrate has its surface
dry-polished using a polishing means including at least one of the
following: chromium oxide Cr.sub.2 O.sub.3, cerium oxide CeO.sub.2,
and iron oxide Fe.sub.2 O.sub.3. The film is then formed on the
silicon carbide substrate. As a result, the mechanochemical
polishing effect of chromium oxide Cr.sub.2 03, cerium oxide
CeO.sub.2, or iron oxide Fe.sub.2 O.sub.3 provides a good polished
surface free of mechanical defects or crystal distortion, and an
epitaxial film can be laminated on the polished surface to
manufacture a good silicon carbide electronic device.
* * * * *